The so-called "wet process" phosphoric acid, produced by the acidification of phosphate rock, contains a significant amount of uranium, typically 0.1 to 0.2 grams per liter. Accordingly, the recovery of uranium from wet process phosphoric acid can provide part of this important nuclear fuel source material supply. It has been demonstrated that the most efficient means of recovering the uranium is by a liquid-liquid solvent extraction. In a publication by the U.S. Atomic Energy Commission, ORNL-TM-2522 (1969), entitled "Solvent Extraction of Uranium From Wet-Process Phosphoric Acid," a process is described in which an aqueous solution of uranium-containing phosphoric acid is contacted with an extractant combination of di(2-ethylhexyl)phosphoric acid (DEHPA) and trioctylphosphine oxide (TOPO) dissolved in an organic diluent. The uranium is then stripped from the DEHPA/TOPO extractant solution by the use of ammonium carbonate. Further refinements of this process are reported in an article entitled "Recovery of Uranium From Wet-Process Phosphoric Acid" by F. J. Hurst et al, Ind. Eng. Chem. Process Des. Develop. 11 (1) 122 (1972) and in U.S. Pat. No. 3,711,591. The process flow sheet disclosed in the latter two references involve a two-cycle extraction. In the first cycle, the uranium-containing wet process phosphoric acid is treated with an oxidizing agent to oxidize the uranium to the hexavalent (U.sup.+6) state and is then contacted with a mixture of TOPO and DEHPA at a 1:4 molar ratio dissolved in a water-immiscible aliphatic hydrocarbon diluent. The uranium is recovered from the organic solvent by a reductive stripping process involving the use of ferrous ion to reduce the uranium to the less extractable tetravalent (U.sup.+4) state. In the second cycle, the uranium is re-oxidized to the U.sup.+6 state and re-extracted with a DEHPA/TOPO solution at 1:4 molar ratio in a diluent and then stripped with ammonium carbonate to precipitate ammonium uranyl tricarbonate (AUT). The latter is calcined to form U.sub.3 O.sub.8.
While the above process has been successfully used on a commercial scale, a need still exists for a more efficient process to recover the relatively small amounts of valuable uranium from the wet process phosphoric acid.
It has now been unexpectedly discovered that enhanced extraction of uranium from wet process phosphoric acid can be obtained with a solvent extractant combination of dialkyl phosphoric acid, e.g. DEHPA, and a mixture of phosphine oxides containing unsymmetrical tertiary phosphine oxides. The phosphine oxide mixture containing at least four components is prepared by the reaction of phosphine and a mixture of two olefins followed by oxidation of the resulting trialkyl phosphine mixture to the corresponding tertiary phosphine oxides. The amount of each component is carefully controlled by the ratio of the two olefins reacted with phosphine in the range of 10:90 to about 90:10 and preferably, in the range of 60:40 to 40:60. The instant process is advantageous in that it provides substantially higher uranium extraction efficiency than the conventional TOPO/DEHPA extraction solvent mixture, thus reducing the number of extraction stages and resulting in increased overall productivity.
The tertiary phosphine oxide mixture used in the process of this invention is disclosed in European Patent Application No. 132700, published Feb. 13, 1985. This mixture is reported to extract acetic acid and phenol from aqueous solutions. It has now been found that a combination of this mixture with dialkyl phosphoric acid could be used to extract uranium from phosphoric acid solutions.